3D Time-of-flight distance measurement with custom - Universität ...

20 CHAPTER 2
range of wavelengths from blue (400 nm) to the near infrared (1200 nm) with an
optical power of up to several milliwatts. Lasers, even laser diodes, are much more
expensive than LEDs and are often larger. However, they offer more optical power
and are suited for operation up to some GHz, also at a wide range of wavelengths
[SAL]. Practically, lasers and laser diodes are the only light sources suitable for
pulsed TOF operation. All other light sources introduced here are actually used for
CW modulated light, due to the reduced bandwidth requirements.
Kerr cells are based on the quadratic electro-optic effect, where the polarization of
a polarized beam is rotated depending on the applied voltage. Together with a
polarizer and a modulated control voltage of the cell, a polarized incoming beam
can be modulated in intensity. Kerr cells can be used as modulators up to 10 GHz
but voltages as high as 30 kV must be switched at this speed [HEC]. Pockels cells,
making use of the linear electro-optic effect (pockels effect), work very similarly and
also require polarized light for operation. Their cut-off frequency of more than
25 GHz is even higher than that of Kerr cells. The driving voltage requirements are
a factor of 10 lower than for an equivalent Kerr cell. However, the Pockels cell still
requires some kilovolts to switch from transparent to opaque mode. Therefore, the
practical cut-off frequency, which is limited by the cell’s capacitance, reaches only
several hundred MHz [HEC, SAL, XU].
Liquid crystal shutters are limited to some kHz of modulation frequency and are
therefore not suited for high-resolution measurement applications in the 10 meter
range [SAL]. An interesting alternative, however, might be the realization of a
mechanical shutter. Miniaturized air turbines with a rotation speed of 427’000 rpm
have been reported [SLO]. If one manages to rotate a light grating disc with such
an air turbine, shutter frequencies as high as 1 MHz might become possible. For
example, a disc of 10cm diameter and a slot width of 1 mm at the edge of the disc
would contain about 150 slots. Such a disc would offer a 1x1 mm 2 shutter aperture
while allowing 1 MHz shutter frequency (=150*427000(1/min)/60(s/min)).
Detectors and Demodulation
The easiest and most straightforward way of realizing a TOF-receiver is to use any
fast and sensitive electrooptical sensor as a detector. The time of flight can then be
determined as follows: a linear analog ramp (or a fast digital counter) is started
synchronously with the transmission of a laser pulse. Once the laser pulse reaches
the detector the rise of the ramp is stopped. The resulting amplitude is then